The invention relates to an electro-optical range finder.
In many geodetic applications, the emission of laser light is required or advantageous. This relates in particular to electro-optical distance measurement with geodetic accuracies, which are typically in the millimeter or submillimeter range and are achievable, for example, by the pulse transit time meter or phase meter principles. Suitable methods and apparatuses of the generic type for distance measurement are described, for example, in EP 0 738 899 B1 or WO2004/074773.
The measurement of distances over the relatively large distances required for geodetic applications sets high requirements with regard to the beam source. For highly accurate distance measurements, it is advantageous if the radiation source provides radiation having a well defined optical pulse shape. For an accurate distance measurement according to the transit time measuring principle, the beam source must therefore be pulsed in the ns range, have a high pulse peak power and have very good beam quality, for example a flat, not curved, emission wavefront.
The requirements resulting from this important field of use with regard to the laser emission of geodetic devices relate to the power and the mode structure. While powers in the mW range are achieved in the case of continuous emission, it is advantageous, for distance measurements over relatively large distances, to achieve powers in the range of a few 10 W, which can be achieved in the pulse mode in particular by short but high-energy pulses. In addition, a beam cross-section which is as small and homogeneous as possible should be provided so that resolution of small structures is also possible. The beam cross-section or the beam profile should as far as possible remain constant over the entire measured distance or should change only slightly.
In geodetic distance measurements of the prior art, laser diodes are frequently used as laser sources. However, these semiconductor lasers have the disadvantage that they emit in multimode operation and, as edge emitters, have a geometrically unfavourable beam cross-section.
Thus, various approaches for converting the emission of a laser source by suitable choice of the laser type, special mode of operation and beam shaping means into a form useable for geodetically precise applications exist in the prior art.
For example, WO 01/84077 discloses an optical range finder which deflects the part-beams of an edge-emitting laser diode through a downstream beam forming optical system and guides them onto the aperture of an objective lens so that they substantially fill said aperture. However, the emission of the laser diode still has a multimode characteristic.
The combination of the emission of many individual laser diodes of an array into a common beam, which combination is also possible for increasing the power, also has the disadvantage of poor coherence.
Commercial narrow-stripe semiconductor lasers available today and having an emission area of 1×3 μm permit diffraction-limited radiation but are suitable only for the pulsed range of less than 1 W pulse peak power with a still acceptable lifetime, e.g. of 5000 h. Higher pulse peak or peak powers lead to irreversible damage to the optical exit facette (catastrophic optical damage, COD).
Broad-stripe emitters designed for a higher power range and having emission widths of 100-500 μm are loadable with regard to the maximum pulse power in less than 1 μsec up to a few 10 watt in the pulse mode but have a very poor radiation characteristic, i.e. multimode operation. The beam shaping by means of a diffractive element or the incorporation into an external cavity permits the optimisation of the radiation characteristic but with considerable effort, for example with regard to the resonator adjustment or with substantially limited quality in comparison with narrow-stripe emitters (diffractive solution).
EP 1 517 415 and WO 2005/029114 disclose, for improving the emission of laser radiation in a geodetic device, a laser source in which the radiation of multimodally emitting laser diodes is influenced by a mode-selective component so that the laser radiation emitted by the laser source has monomodal character. For this purpose, it is proposed to operate an edge emitter or a vertical semiconductor emitter with an external cavity in which a mode-selective component is present, for example a monomodal fibre or resonator mirror, which result in a mode-selective resonator structure. For compensation of the greater pulse duration achieved by the lengthened cavity, components having a negative dispersion can be used for pulse compression.
In addition, it is possible in principle also to use other laser types, for example microchip solid-state lasers in single-mode operation, pumped by semiconductor lasers, in geodetic devices. However, these have the disadvantage of large dimensions, high energy consumption and an unfavourable operating characteristic, for example owing to thermal effects. As a result, the suitability of such solutions in practice for use for surveying in the field is limited.
Diode-pumped solid-state lasers meet the requirements with regard to the beam quality and the peak power but are very complicated with respect to the concept (pumped laser with actuation, coupling-in optical system, amplification medium, quality circuit, resonator) and expensive. They also generally permit flexible adaptation of the pulse rate since this is a limiting factor due to the high thermal load in combination with lower efficiency.